The production of steroid hormones by the mammalian preimplantation embryo and the physiologic use of these hormones for embryonic development are presently controversial, but nevertheless important concepts. Two basic problems are involved in resolving the controversies. First, most of the available evidence supporting the idea that preimplantation embryos synthesize steroids is circumstancial and inconclusive, However, there is no conclusive contradictory evidence to negate the concept either. Secondly, the idea that steroid hormones play a vital role in embryonic development and implantation of the blastocyst need not depend upon embryonic steroidogenesis, i.e., preimplantation embryos need not produce steroids though they may require them. The objectives of the proposed research are aimed at these two fundamental but separate concepts. Experiments are designed to answer the question whether preimplantation embryos synthesize steroids or not. On the other hand, studies regarding function of steroids in preimplantation phenomena are included. Experimental methods to be employed include culture and transplantation of preimplantation embryos, histochemical and biochemical assays of enzymes, radioimmunoassay of steroids, etc.